This invention relates generally to apparatuses used in the treatment of natural gas. More particularly, the invention relates to semi-permeable membranes used in natural gas separation processes to remove acid gas and other components from a natural gas stream so that the gas may be usable for fuel.
Much of the world's produced natural gas contains unacceptably high concentrations of acid gas—primarily CO2 and H2S—which must be removed before the gas is usable for fuel. The utilization of semi-permeable membranes for CO2/natural gas separation is well known and spiral wound and hollow fiber membrane configurations have been used for this purpose.
Commonly used membrane configurations include fiber materials made of organic polymers and copolymers including polysulfones, polycarbonates, cellulose acetates, cellulose triacetates, polyamides, polyimides, and mixed-matrix membranes. Regardless of the fiber material used, the membrane elements are fabricated with the same type of fiber material throughout. Therefore, these membranes are limited to a single and set range of performance characteristics even though gas properties and volumes change throughout the membrane as permeation occurs. Additionally, current technology may require the operation of the membrane elements in multiple stages, so that gas is passed through multiple groups of membranes in series to partially compensate for the inefficiencies in the performance of each individual membrane stage. The result is additional equipment requirements and less than optimal membrane separation performance.
For applications that require a significant amount of CO2 removal, membrane element performance can be restricted by these uniform performance characteristics. For example, a membrane fiber that performs well in higher CO2 conditions may be less effective at lower CO2 concentrations (and vice versa). As a result, system design is often based on a compromise limited by the performance characteristics of the membrane fiber. Because of less than optimal membrane separation performance, additional equipment and additional stages of membrane elements are required to remove high percentages of CO2. In addition, the membrane separation performance achieved with a single fiber type may be less efficient overall, resulting in higher hydrocarbon losses to permeate.
In many applications, the inlet gas has a high percentage (generally 10-95%) of inlet CO2 and membrane elements are used to bulk remove CO2. As discussed previously, for high CO2 applications, the membrane elements often are configured to operate in series with multiple stages of membranes in operation. This can result in an inefficient configuration for equipment, which requires interconnect piping between stages, thereby creating a larger overall equipment footprint and higher equipment cost. Having multiple stages of membranes may also result in difficulty in balancing the flow rates and CO2 removal duties for each stage of membranes, as the amount of membrane surface area installed in each stage may have to be individually adjusted in order to maintain the desired separation performance characteristics.
Recent improvements in membrane manufacturing have led to significant increases in membrane fiber surface area in a single membrane element. For example, FIG. 1 shows older, prior art 5-inch and 12-inch diameter CYNARA® membrane elements 10 (Cameron International Corporation, Houston, Tex.) which have 500 and 2,500 square feet of active membrane fiber area, respectively. In comparison, newer larger 16-inch and 30-inch diameter membranes have been developed that have between 9,000 and 40,000 square feet of active fiber area, respectively. In the prior art—and unlike those made according to the invention disclosed herein—these larger diameter membranes are single fiber type membranes. The shear surface area of these larger diameter membranes provides greater capacity and allows for fewer stages of processing when compared to the number of processing stages needed when smaller diameter membranes are used. However, these larger membranes experience a larger gradient in, for example, CO2 concentration between the inlet and outlet side of the membrane when compared to the smaller diameter membranes. This larger gradient can reduce the effectiveness of these larger diameter, single fiber type membrane elements.
For membrane gas separation applications, the relative composition of the gas changes as the gas travels through the membrane bundle and permeable components are separated from the non-permeate components. At the same time, the inlet to non-permeate gas volume is reduced as gas passes through the membrane bundle and permeation occurs, with the inlet gas first entering the membrane being higher in volume and permeable components than the non-permeate gas exiting the membrane bundle. In other words, the gas oftentimes undergoes significant and non-uniform compositional changes as it travels through the membrane. Therefore, a need exists for a membrane element that has the requisite performance characteristics for improved gas separation even as the gas volume and composition change as the gas travels through the membrane.